It has been known in the art that compounds containing imide group provide excellent heat resisting characteristics. It is also well-known in the art that polyimide resins exhibit superior heat resisting properties. Polyimide resins also provide excellent mechanical strength, in additional to their superior heat resisting properties. These advantageous properties thereof enable polyimides to be widely used in the industry. A typical example of the applications in which polyimides are extensively used is the production of printed circuit boards (PCBs), especially flexible PCBs. Polyimides are also widely used as varnishes for coating electric wires and in the making of packaging materials for the electronic/electrical industry.
One of the disadvantages of polyimides is their inferior solubility in most organic solvents. This solubility problem presents a barrier in attempting to directly applying the solution processing technology for fabricating polyimide products. In most applications where polyimides are utilized, a precursor polymer--most commonly a polyamic acid--is used instead of directly using the polyimide resin. After the fabricating steps, polyamic acid is then cyclized to become polyimide. The storage of polyamic acid prior to processing often causes a ponderous problem in the polyimide processing industry. This problem is aggravated by the strong corrosiveness of the polyamic acid. Furthermore, the need to dewater the final product after cyclization also adds difficulty to the polyamic-to-polyimide indirect solution processing approach. While this practice might have been acceptable in the past, it could raise serious problems as the need to mitigate environmental pollution is becoming a global concern.
In addition to poor solubility in organic solvents as described hereinabove, polyimides also exhibit relatively inferior transmissibility to visibly light rays. Such a poor transparence of polyimides is related to the frequency of molecular vibration of the polyimide molecules. In many industrial applications, it is desirable to provide a heat-resisting polyimide that is also highly transparent.
In U.S. Pat. No. 3,426,098 (the '098 patent), it is disclosed a polyester-polyimide prepared from the reactants of: (1) tris (2-hydroxyethyl) isocyanurate; (2) a polycarboxylic acid, such as terephthalic acid or isophthalic acid; (3) an aromatic diamine, such as oxydianiline or methylene dianiline; and (4) an aromatic carboxylic anhydride containing at least one additional carboxyl group, such as trimellitic anhydride or pyromellitic anhydride. Because of the existence of the fatty ester linkages, the polyester-polyimide resins prepared according to the '098 patent exhibit relatively inferior heat resisting property.
In U.S. Pat. No. 3,697,471 (the '471 patent), it is disclosed a polyesterimide resin produced from dimethyl terephthalate, ethylene glycol, glycerin, and a 5-membered imide ring compound. The 5-membered imide ring compound was produced from reactions of an aromatic carboxylic acid anhydride, which, besides the 5-membered cyclic carboxylic acid anhydride group, also contains at least one additional reactable group; and a primary amine, which, besides the primary amino group also contains at least one additional reactable group. Since the polyesterimides prepared according to the '471 patent also contain fatty ester linkages, they similarly exhibit relatively inferior heat resisting property.
The processes disclosed in both the '098 and the '471 patents require a phenolic solvent (cresol in the '098 patent and a mixture of p-chlorophenol, phenol and o-cresol in the '471 patent). The offensive odor of the phenolic solvents and their adverse effects on human skins could cause significant processing problems. The polyesterimide resins prepared according to either the '098 or the '471 patent show brownish color; they both exhibit inferior transmission to visible light rays. Furthermore, in order to achieve solubility in the solvents mentioned hereinabove, the polyesterimides disclosed in the '098 and the '471 have relatively low molecular weight. As a result, these resins have relatively poor filmability, poor mechanical strength, and are unsuitable for use as engineering plastics.
U.S. Pat. No. 4,855,390 discloses a polyesterimide resin obtained by the reaction between a bis-(hydroxyphthalimide) and a dicarboxylic acid dihalide. The bis-(hydroxyphthalimide) is prepared by reacting 4-hydroxyphthalic anhydride with an organic diamine such as m-phenylenediamine. The dicarboxylic acid dihalide is obtained by the halogenation of a dicarboxylic acid such as terephthalic acid. The polyesterimide resin prepared according to the '390 patent was reported to exhibit good heat resistance and light transmittance. However, the '390 patent did not address many of the important issues that have significant bearings on the commercial applicability of polyimide such as solubility, filmability, mechanical strength, and molecular weight thereof. Bis-(hydroxyphthalimide) is not a commercially available product and the polyesterimide of the '390 patent requires a two-step process.